Frederick became king on his father's death in early 1947. During Frederick IX's reign, Danish society shook off the restrictions of an agrarian society, developed a welfare state and, as a consequence of the booming economy of the 1960s, women entered the labour market. Denmark modernized, bringing new demands on the monarchy and Frederick's role as a constitutional monarch. Frederick IX died in 1972, and was succeeded by his eldest daughter, Margrethe II.

Christian IX died on 29 January 1906, and Frederick's grandfather Crown Prince Frederick succeeded him as King Frederick VIII. Frederick's father became crown prince, and Frederick moved up to second in line to the throne.

Frederick was educated at the Royal Danish Naval Academy (breaking with Danish royal tradition by choosing a naval instead of an army career) and the University of Copenhagen. Before he became king, he had acquired the rank of rear admiral and he had had several senior commands on active service. He acquired several tattoos during his naval service.

In addition, with his great love of music, the king was an able piano player and conductor.

Frederick IX's reign saw great change. During these years, Danish society shook off the restrictions of an agricultural society, developed a welfare state, and, as a consequence of the booming economy of the 1960s, women entered the labour market. In other words, Denmark became a modern country, which meant new demands on the monarchy.

As King Frederick IX and Queen Ingrid had no sons, it was expected that the king's younger brother, Prince Knud, would inherit the throne, in accordance with Denmark's succession law (Royal Ordinance of 1853).

However, in 1953, an Act of Succession was passed, changing the method of succession to male-preference primogeniture (which allows daughters to succeed if there are no sons). This meant that his daughters could succeed him if he had no sons. As a consequence, his eldest daughter, Margrethe, became heir presumptive. By order of 27 March 1953 the succession to the throne was limited to the issue of King Christian X.

Shortly after the King had delivered his New Year's Address to the Nation at the 1971/72 turn of the year, he became ill with flu-like symptoms. After a few days rest, he suffered cardiac arrest and was rushed to the municipal hospital on 3 January. After a brief period of apparent improvement, the King's condition took a negative turn on 11 January, and he died 3 days later, on 14 January, at 7:50 pm surrounded by his immediate family and closest friends, having been unconscious since the previous day.[7][8]

He was succeeded by his eldest daughter, Queen Margrethe II.[12] Queen Ingrid survived her husband by 28 years. She died on 7 November 2000. Her remains were interred alongside him at the burial site outside Roskilde Cathedral.

On 20 April 1982, a statue of King Frederick IX dressed in the uniform of an admiral was unveiled by the Copenhagen harbour on the 35th anniversary of his accession to the throne in 1947 and in the tenth year after his death.[13]

Princes that lost their title following an unequal marriage are shown in italics^Not a Danish prince by birth, but created a prince of Denmark*Also a prince of Greece ∞Also a prince of the United Kingdom ‡Also a Count of Monpezat